The invention relates to glasses with a refractive index close to 1.7, an Abbe number close to 35 and a density close to 3.2.
Many glasses of varying compositions have been disclosed in the past having similar properties, particularly for use in optical and ophthalmic applications. However, these glasses have presented several disadvantages such as high batch cost, low viscosity at the liquidus temperature, low chemical durability and/or yellow coloration.
For example FR-2 427 309 describes glasses having the following compositions, expressed in terms of weight percent on the oxide basis:
______________________________________ SiO.sub.2 40-46 Al.sub.2 O.sub.3 0-3 SiO.sub.2 + Al.sub.2 O.sub.3 40-46 TiO.sub.2 13-17 ZrO.sub.2 4-11 Nb.sub.2 O.sub.5 2-10 BaO 8-16 SrO 0-4 CaO 3-7 MgO 0-2 BaO + CaO + SrO 15-25 CaO + MgO 4-8 Li.sub.2 O 0-3 Na.sub.2 O 2-7 K.sub.2 O 3-8 Li.sub.2 O + Na.sub.2 O + K.sub.2 O 5-12. ______________________________________
In addition to having a refractive index of about 1.7, an Abbe number of about 35, and a density in the region of 3.2, these glasses exhibit good resistance to acids and viscosities at the liquidus in excess of 22 Pa.s, which is advantageous for their production, allowing their direct molding. These properties make such glasses particularly suited for optic and ophthalmic applications. However, the high Nb.sub.2 O.sub.5 content (2-10%) of the glass compositions make the cost of these glasses rather high.
Accordingly, it is the object of the present invention to provide low cost glasses having similar properties to those described above, and having high resistance to devitrification.